A power MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is a semiconductor device containing two regions of a given conductivity type, known as the source region and the drain region, separated from each other by a region of opposite conductivity type, known as the body region. Charge carriers of the given conductivity type can flow between the source and drain through a gate-controllable channel of the given conductivity type extending across a surface of the opposite conductivity type body region. The gate is an electrically conductive electrode situated over an electrically insulating layer of oxide over the surface which contains the channel. Conductivity of the channel, and hence the current flowing between source and drain regions, is changed as changes are made in the voltage at which the gate is biased relative to the body region. The device is known as an N-channel or P-channel MOSFET in accordance with the conductivity type of the source, drain, and channel regions. The device is known as an enhancement-mode or depletion-mode MOSFET in accordance to whether an applied gate bias produces an enhancement or depletion of the conductivity of the channel. An enhancement-mode device is normally off, and does not conduct current unless a bias voltage is applied to the gate. A depletion-mode device is normally on, and will conduct current when no bias voltage is applied to the gate, but this current flow can be interrupted by a bias voltage applied to the gate.
A typical N-channel enhancement-mode MOSFET structure is shown in FIG. 1, and its electrical characteristics are shown in FIG. 2. The device contains an N type source region and an N type drain region, separated from each other by a P type body region. The source, body, and drain regions all intersect a first surface of the structure, while only the drain region intersects a second surface of the structure. A first-surface metal makes contact to the source and body regions on the first surface, and a second-surface metal makes contact to only the drain region on the second surface. The presence of the junction between P body region and N drain region prevents flow of current from second-surface metal to first-surface metal, but the presence of the contact of first-surface metal to P body allows current to flow in the opposite direction, i.e. from first-surface metal to second-surface metal. Overlying the P body region on the first-surface, but separated from it by a layer of insulating oxide, is an electrically conductive gate. When a positive bias is applied to the gate, an N type channel consisting of electron current flow is induced along the surface of the P body region, allowing current to flow between the two N regions. In this way, a small voltage applied to the gate terminal, relative to the N type source potential contacted by first-surface metal, can control a high current at a high voltage between the first-surface and second-surface metals.
Current flowing in a semiconductor can consist of negative charge carriers (electrons), or positive charge carriers (holes), or both. Unipolar conduction consists of only one species of charge carrier, and bipolar conduction consists of both species. Conduction in an N channel MOSFET is unipolar, since it consists of the flow of only electrons.
An N-channel enhancement-mode IGBT (Insulated Gate Bipolar Transistor), is shown in FIG. 3, and its electrical characteristics are shown in FIG. 4. It is structurally identical to the MOSFET, with the exception that the heavily-doped N type drain contact region at the second-surface of the device has been replaced by a heavily-doped P type region. This second-surface P region functions as a P emitter which injects holes into the N type base region. The base of a IGBT is the wide center region of a IGBT in which a wide depletion layer forms to allow a large blocking voltage to be established without avalanche breakdown in the IGBT with the IGBT is in the off-state and a large value blocking voltage is established across the first-surface and second-surface metal terminals of the IGBT. As a result, the IGBT has bipolar conduction, which is a current flow consisting of both holes and electrons in the base, rather than unipolar conduction consisting of a flow of only electrons, as in the N-channel MOSFET. The mobile negative charges (electrons) are balanced by the mobile positive charges (holes), so the carrier concentration in the IGBT can build up to a value several orders of magnitude higher than in the MOSFET through the mechanism of conductivity modulation, with a corresponding reduction in the resistance to current flow, and a corresponding increase in current per unit area. This ability to function at a significantly higher current per unit area is the primary motivation for using an IGBT rather than a MOSFET in any given application.
An N-channel enhancement-mode DIGBT (Double-sided Insulated Gate Bipolar Transistor), is shown in FIG. 5, and its electrical characteristics are shown in FIG. 6. This is structurally identical to the MOSFET and IGBT, with the exception that the heavily-doped N or P type region at the second surface of the device has been replaced by a second MOSFET structure at the second surface. Like the IGBT, the DIGBT can have bipolar conduction in the base because the P body regions of the lower MOSFET at the second surface can act as a P emitter regions, injecting holes into the base region. But unlike the IGBT, the DIGBT can also have unipolar conduction, because when the second-surface gate of the MOSFET at the second surface is turned on, it provides an N type channel path through which the electrons can flow and bypass the second-surface P regions. In addition, when the second-surface gate of the MOSFET at the second surface is turn on, it brings the potential of the N-type base to the potential of the N type source at the second surface thereby turning off the PN junction formed by the P body region and the N-base and halting the hole injection from the P body region at the second surface.
The first surface gate of the DIGBT is the primary gate and the second surface gate of the DIGBT is the secondary gate. The primary gate of the DIGBT controls the turn-on and turn-on of current flow in the channel in the DIGBT by controlling the turn-on and turn-off of the MOSFET at the first surface that injects majority carriers into the base of DIGBT. For an DIGBT with an N-type base region, the primary gate is on the same side of the DIGBT as the cathode terminal. The base of a DIGBT is the wide center region of a DIGBT in which both hole and electron current flow occurs to enable conductivity modulation and a wide depletion layer forms to allow a large blocking voltage to be established without avalanche breakdown in the DIGBT with the DIGBT is in the off-state and a large value blocking voltage is established across the terminals of the DIGBT. The secondary gate is the gate of the DIGBT that controls the injection of minority carriers into the base of the DIGBT. For an DIGBT with an N-type base region, the secondary gate is on the same side of the DIGBT as the anode terminal.
For a given voltage-supporting capability, all 3 devices have the same structure on the first surface, and the same thickness and dopant concentration in the voltage-supporting layer. The difference between the 3 devices lies in the structure of the second surface. In the MOSFET, the second-surface metal makes contact to material having the same conductivity type as the base. In the IGBT, the second-surface metal makes contact to material having the opposite conductivity type to that of the base. In the DIGBT, the second-surface metal makes contact to either the same or the opposite conductivity to that of the base, depending on the status of the second-surface gate.
FIGS. 2, 4, and 6 show the electrical characteristics of these 3 types of devices. Electrical characteristics in the 1st quadrant (the direction in which, on an N-channel device, the second-surface metal is biased positively with respect to the first-surface metal) are governed primarily by the first-surface structure. Since all 3 devices have the same first-surface structure, all 3, when the channel is turned off, have the same 1st quadrant electrical characteristics. Electrical characteristics when the channel is turned on, and the electrical characteristics in the 3rd quadrant, are governed by the second-surface structure, and so are different on these 3 different devices.
FIG. 2 shows the electrical characteristics of an N-channel enhancement-mode MOSFET. With no gate bias applied, the device has the same 1st quadrant blocking characteristics as the IGBT and DIGBT, but in the 3rd quadrant, it supports only the 0.5 to 1 volt of the forward-biased PN junction between body and drain regions. With a positive gate bias applied, the induced N channel produces a continuous N-type conductive path between the metal electrodes of the first and second surfaces, so the electrical characteristic shows an ohmic path which is continuous through the origin. Current can flow in the 1st quadrant only when the gate bias is applied. This current is unipolar, consisting of only electrons. Current can flow in the 3rd quadrant regardless of whether a gate bias is applied. Without a gate bias, the 3rd quadrant current is bipolar. With a gate bias, the 3rd quadrant current is unipolar at low current levels, but can become bipolar at higher current levels if the lateral voltage drop within the device becomes high enough to exceed the 0.5 to 1 volt needed to forward-bias the body-drain junction.
FIG. 4 shows the electrical characteristics of an N-channel enhancement-mode IGBT. With no gate bias applied, this device blocks the flow of 1st quadrant current, just as does the MOSFET. But in the 3rd quadrant, it also blocks the flow of current because the PN junction, which has been added at the second surface, is now reverse-biased. No 3rd-quadrant current flows until the breakdown voltage of this second-surface, 3rd-quadrant, junction is exceeded. Breakdown voltage of the 3rd-quadrant junction is independent of that of the 1st-quadrant junction, and can be made equal to, less than, or greater than the other by changing local dopant concentrations of voltage-supporting regions. Breakdown voltages can be designed to meet the 1st and 3rd quadrant requirements of the intended application. When a positive bias is applied to this IGBT gate, the induced N channel connects the N source to the N drain region just as it does in a MOSFET. But since the second-surface junction is in series with this path, the 1st quadrant current flow is offset from the origin by the voltage drop of this forward-biased second-surface junction. Due to the injection of holes from this junction, the resulting 1st-quadrant current flow in the IGBT is bipolar, and so has a considerably lower voltage drop than does the unipolar 1st-quadrant current of the MOSFET. In the 3rd quadrant, the IGBT blocks the flow of current regardless of whether or not a gate bias is applied.
FIG. 6 shows the electrical characteristics of an N-channel enhancement-mode DIGBT. This device has a first gate (gate 1) on the first surface and a second gate (gate 2) on the second surface, so the electrical characteristics are a function of the status of both gates. When both gates are at zero or negative bias, the DIGBT blocks the flow of current in both directions, up to the avalanche breakdown voltage. As in the IGBT, the breakdown voltage of the 3rd-quadrant junction is independent of that of the 1st-quadrant junction, and can be made equal to, less than, or greater than the other by changing the local dopant concentrations, and can be designed to meet the requirements of the intended application.
When a positive bias is applied to gate 1 while gate 2 is held at zero, the electrical characteristics of the DIGBT are identical to the IGBT: bipolar current flows in the 1st quadrant, but no current flows in the 3rd quadrant. When a positive bias is applied to gate 2 while gate 1 is held at zero, the electrical characteristics are identical to an IGBT in the opposite direction: bipolar current flows in the 3rd quadrant, but no current flows in the 1st quadrant.
When positive biases are applied to both gates of the DIGBT, current can flow in both directions, but this current becomes a unipolar flow, like a MOSFET, rather than a bipolar flow. The current becomes unipolar because the N channel on one surface allows only electrons to enter, and the N channel on the opposite surface allows those electrons to exit without having to cross a junction, and hence without injecting any holes. Actually, at very high current levels, there may be some hole injection because the lateral voltage drops within the device may become high enough to locally forward-bias some of the PN junctions. The current level at which this occurs will depend on the geometry and dopant concentrations of the particular device.
This ability of the DIGBT to conduct by either bipolar or unipolar flow is the reason the DIGBT has advantages over MOSFETs and IGBTs. Bipolar conduction allows current to flow with a much lower voltage drop, less-power dissipation, than unipolar conduction. This is why current per unit area capability of an IGBT is much higher than of a MOSFET. But bipolar conduction leaves the device with a quantity of stored minority carrier charge in the base which must be removed before the device can interrupt, or turn off, the current flow, so this is why IGBTs are slower to turn off and have more switching dissipation than MOSFETs. The advantage of the DIGBT is that through appropriate control of the gate biases, it can conduct like an IGBT and yet can turn off like a MOSFET. This is accomplished by turning on only one gate (the primary gate) while the device is conducting, then turning on both gates (the primary and secondary gates) for a short time just prior to turning off the device (by turning off the primary gate). While only one gate is on, there is bipolar current flow with its low voltage drop, but when both gates are turned on, this changes to a unipolar flow. The voltage drop increases as the carrier concentration drops to the unipolar level, but this is for only a short time just before turn-off, and the reduced minority carrier concentration in the base results in a faster turn-off and reduced switching dissipation, for a net reduction in power losses in the device.
Commonly-used terminology for the MOSFET refers to the first-surface metal terminal as the Source, the second-surface metal terminal as the Drain, and the control electrode as the Gate. Commonly-used terminology for the IGBT refers to the first-surface metal terminal as the Emitter, the second-surface metal terminal as the Collector, and the control electrode as the Gate. Terminology for the DIGBT is not yet well-established, but an appropriate system seems to be to refer to the first-surface metal as Terminal 1, second-surface metal as Terminal 2, and the control electrodes as Gate 1 and Gate 2. Terminal 1 is the reference terminal for Gate 1 (primary gate), and Terminal 2 is the reference terminal for Gate 2 (secondary gate). When the voltage-blocking capability is higher in one direction than the other, Gate 1 (primary gate) is the gate which controls current flow in the higher-voltage direction.
FIG. 7 shows an individual phase-leg, also known as a half-bridge, a basic circuit in which MOSFETs, IGBTs, and DIGBT's can be used. Also shown in FIG. 7 is a single-phase bridge, made up of two phase-legs.
FIG. 8 shows a 3-phase bridge, made up of 3 phase-legs. The individual phase-leg (half-bridge) consists of two switches, referred to as switch A and switch B, and two rectifier diodes, referred to as diode A and diode B. The switches could be any of the 3 devices being discussed, or any other switching device. Phase-legs can be connected together in various types of bridge circuits to produce variable-voltage, variable-frequency, single-phase or multi-phase AC power for electric motors or any other high-power loads.
In operation, the DC supply is connected to opposite ends of the legs, and the load is connected between centers of the legs. In the single-phase bridge of FIG. 7, voltage is applied to the load by simultaneously turning on switches A & D, or B & C. Direction of the load current is determined by which pair of switches has been turned on. The AC load frequency is governed by the rate at which load current direction is being changed from pair A & D to pair B & C and back.
While current is flowing through a given pair, say pair A & D for example, the magnitude of the voltage being applied to the load is controlled by rapidly turning the pair A & D on and off, at a frequency which is high compared to the AC frequency being applied to the load. This may be referred to as the “drive” mode for pair A & D, because during this time, the voltage on the load is established by the ratio of on-time to off-time of the A & D pair. While pair A & D is in the drive mode, pair B & C is in what may be called the “freewheeling” mode. During the time pair A & D is on, energy is being transferred from the supply to the load, and during the time pair A & D is off, energy is being transferred from the load back to the supply by means of current flowing through the diodes of pair B & C.
FIG. 9 shows the current and voltage waveforms for an example of the operation of a phase-leg circuit. In this example, the circuit is being switched at a rate which applies a quasi-sinusoidal alternating current to an inductive load, such as an electric motor. With this type of circuit, motor speed can be varied by varying the AC frequency, and torque can be varied by varying the voltage through changes in the on-time to off-time ratio.
When a given switch-plus-diode set in a given leg is in the drive mode, this set is alternating between 1st-quadrant conduction and 1st-quadrant blocking, at a rate determined by the gate signals being applied to this given set. Meanwhile, the other set is in the freewheeling mode.
When a given switch-plus-diode set in a given leg is in the freewheeling mode, this set is alternating between 1st-quadrant blocking and 3rd-quadrant conduction, at a rate determined by the gate signals being applied to the other set. Meanwhile, the other set is in the drive mode.
When switch A is conducting during the drive mode, the 1st quadrant current through switch A is flowing through the load, and the full supply voltage is applied to the load, to switch B, and to diode B. To control the voltage on the load, switch A is momentarily turned off. When this happens, the current through switch A stops, but the load inductance keeps the load current flowing by generating enough Ldi/dt voltage, in the 3rd quadrant, to exceed the supply voltage and push this current back into the supply, through diode B, which becomes forward biased by the load-generated voltage.
FIG. 10 shows a phase-leg using only MOSFETs, and the corresponding waveforms. When MOSFETs are used as the phase-leg switches, no external diodes are needed, because as described above and as shown in the MOSFET electrical characteristics, the MOSFET itself conducts in the 3rd quadrant, and thereby performs both the switching and the freewheeling functions. When IGBTs are used as phase-leg switches, as shown in FIG. 11, they require external freewheeling diodes because IGBTs have no 3rd quadrant conduction.
Power losses in phase-leg circuit operation fall into 3 categories: (1) conduction losses, due to the voltage drop on the switch while it is conducting, (2) turn-on losses, due to the current-voltage-time product as the switch makes the transition from off-state to on-state, and (3) turn-off losses, due to the current-voltage-time product as the switch makes the transition from on-state to off-state.
Reduction of the conduction losses is the primary motivation for using IGBTs or DIGBTs rather than MOSFETs in phase-legs. Conduction losses in the IGBT and DIGBT, which have bipolar conduction, are lower than in the MOSFET, which has only unipolar conduction. However, turn-off losses are higher for the IGBT and DIGBT due to the longer turn-off time caused by the bipolar conduction.
Turn-on losses are primarily a function of the recovery time of the freewheeling diode, because like the IGBT and DIGBT, the diode also has bipolar conduction, regardless of whether it is an external diode or a built-in diode (body diode) of a MOSFET. While the switch is off, the diode is carrying the freewheeling current through bipolar conduction, and is accumulating a stored charge. When the switch again turns on, direction of current through the diode abruptly reverses and flows without impedance until the diode loses its stored charge and recovers its blocking capability. During this interval, the switch and diode are both highly conductive, and are in series directly across the supply with very little impedance to limit the current. Therefore the current of the diode adds to the current of the MOSFET that is turning on and the total current can rise to values higher than the load current and can cause a high dissipation of energy in the switch that is turning on as well as in the diode. As the diode does recover, the rapidly rising voltage on it can also sometimes cause damage to dielectrics.
Conduction losses and turn-on losses can be reduced by use of an IGBT+MOSFET phase-leg as shown in FIG. 12. In this phase-leg, the diodes have been replaced by MOSFETs. As noted above, MOSFETs have built-in diodes, which can perform the freewheeling function. If these MOSFETs are used purely as ordinary diodes, they have the same bipolar conduction, recovery time, and turn-on losses as ordinary diodes. But the MOSFET diodes have an advantage in that when their gate is turned on during diode conduction, the conduction changes from bipolar to unipolar. Therefore, if the gate remains off during most of the freewheeling conduction, but is turned on just before the freewheeling conduction is to be ended, the stored charge and recovery time can be reduced, and the turn-on energy dissipation can be reduced. Conduction losses of the IGBT+MOSFET can be slightly less than the IGBT+diode because the MOSFET can be turned on during the drive-conduction part of the cycle, allowing the MOSFET to carry possibly 10 to 20% of the load current. Turn-off losses can also be reduced in this type of phase-leg, because the MOSFET can be used to reduce turn-off dissipation. This can be done by allowing the MOSFET to remain on beyond the turn-off of the IGBT gate, thereby minimizing the rise in voltage while the IGBT is losing its stored charge and recovering its blocking capability. Then when the MOSFET is turned off, current is cut off more rapidly, resulting in a smaller current-voltage-time product.
Reduction of turn-off losses has been the primary motivation for using DIGBTs instead of IGBTs. To accomplish this, DIGBTs could be directly substituted for the IGBTs in the circuits of FIGS. 11 and 12, and adding the appropriate drive for gate 2. FIG. 13 shows a phase-leg in which the IGBTs of the FIG. 11 circuit have been directly replaced by DIGBTs. As noted above, conduction in the IGBT is always bipolar, while in the DIGBT it can be unipolar or bipolar, depending on the gate biases. When the DIGBT is used as in FIG. 13, it has only gate 1 turned on during most of the conduction cycle, and during this time, it has bipolar conduction. Gate 2 is then turned on just before gate 1 is to be turned off. With both gates on, conduction changes from bipolar to unipolar, so there is less stored charge remaining when gate 1 is turned off, resulting in a shorter turn-off time and less turn-off energy dissipation. DIGBTs could also be substituted for the IGBTs in the circuit of FIG. 12, in which the turn-on losses were reduced by replacing the diodes with MOSFETs.
It is desirable to employ a circuit architecture that further reduces conduction and turn-off losses while simultaneously eliminating the need either diodes or MOSFETs.